Hello fellow Mutt owners, If you are having trouble with your fuel gauge fluctuating all over the place this fix may be for you. When my fuel gauge starting doing this, I replaced the fuel gauge sending unit in the tank. At the time of replacement, the fuel gauge would not read full anymore, even with a full tank, but the fluctuation was gone. I purchased another nos sending unit, and this one was slightly better, but still would not read a full tank. Adjusting the stops on the sending unit, or bending the float arm would still not make it read a full tank. A resistance check with a ohmmeter revealed that the 2 nos units (apparently reproductions, not original quality) do not have the proper resistance. The gauge needs a high resistance around 32 ohms to read full and a low resistance .5 ohms to read empty. This is measured from the connector on the sending unit to sending unit case ground (where the mounting screws hold the sending unit).

The resistance wire, is not the correct value or not enough wire on the form inside the sending unit, not much can be done about that. So most of the summer the mutt had a fuel gauge that did not read full (about 3/4 on the gauge). About 3 months ago the new sending unit started doing the same thing as the original sending unit, fluctuating all over the place and showing a full tank when the tank was almost empty. I pulled the sending unit out of the tank and sure enough an ohmmeter checked showed the resistance was going very high and losing connection when the float arm was moved.

If you look inside the sending unit you can see how it works, the float arm moves the wiper on the variable resistor (the half moon shaped thing with the wire wrapped around it). The float arm (and the wiper contact that is attached to it) needs a ground to send the proper signal to the fuel gauge. Can you see how it gets a ground? The silly wiper gets a ground from the body of the sending unit. After several months in the crap fuel we get today, the wiper no longer makes proper contact with the frame. This can be checked easily with an ohmmeter (wiggle the float rod and watch the resistance go crazy). I cannot believe there is no kind of contact brush or wire to provide a return path for the movable contact. So now I am batting 1,000, 3 sending units and they all suffer from various aliments.

Here is what I did to solve the problem. Starting with the original high quality sending unit (the one that would read full with the proper oem resistance), I decided to supply a ground for the movable contact and float arm. See the pictures below. This repair will work only if you wiper contact is still in good shape, and there is no corrosion on the variable resistor. The wiper should make a slight scraping noise as it wipes against the resistor, confirming proper mechanical contact. The white wire is soldered from the float lever to the support frame Clean the metal well to allow the solder to stick well). I used a weller 150 watt gun.

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The wire is very fine wire used for model aircraft electrical systems. Some people call this litz wire. It is fuel proof and gasoline will not dissolve the insulation. It is wound into a hairspring arrangement so it will not prevent the float from moving freely. It is very important to use the thinnest multi strand wire you can find. From the pictures you should be able to see how it was soldered. You also need to make sure the rivets that hold the top of the sending unit to the rod that supports the float assembly are making a good connection. I found out my sending unit was not making a good connection there either (when the support frame was moved the resistance was intermittent). I soldered the rivets in the photo below. I soldered most of them, but only a couple should be enough if the top of your tank is clean as well as your sending unit mounting screws. After soldering the rivets, file the solder smooth so the sending unit will get a good seal on the gasket. The only thing that could stop the sending unit from reading intermittent now would be your ground on your gas tank. I was going to add a separate ground for the tank, but apparently it is getting a good enough ground through the gas tank mounting screws.

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I have driven the mutt for several months now with the oem sending unit (about 20 years old) and have had no trouble with the gauge fluctuating and fluttering. And it reads empty when empty and full when full. You can connect the sending unit to the gauge for testing before dropping it in the tank, just provide a ground with a alligator clip lead to the body of the vehicle (not near the open fuel tank). You can then move the float up and down and make sure the gauge is going from full to empty while moving the float. If the gauge does not go all the way to full or empty, there are float lever stops you can bend to correct for that. It is possible that the float arm may need to be bent too, but my oem reads full when full. Oh yea, A mutt gas tank is only full when the fuel is so high in the fuel filler neck, that it is about to spill out!!! The fuel filler neck, IS the gas tank! I hear guys tell me all the time they cannot get the proper amount of fuel capacity in the tank, this is why. It takes much patience at the gas pump to fill it up that high, but your gauge will read full if you do. If your cap seals good, gas usually does not spill out off road or on right turns.

I feel bad on pissing away close to $90 worth of “junk” reproduction sending units. They were purchased from 2 different sources too. The numbers on the original sending unit cover are MS-500040-2 57733 8376496. The replacements do not have any numbers on the cover, something to be careful of. If anyone can locate a source for the “Real Mcoy” OEM sending unit, I would love to hear from you, the reproductions are junk with the wrong electrical resistance. So hopefully this information will allow you folks to repair your original sending unit and save yourselves some $$$ in the process.

09:24, 22 December 2005 (PST)Louie,

MVPA 27368

M151A2

 
fuel_gauge_sending_unit.txt · Last modified: 2008/05/05 11:45 by Horst
 
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